Oxygen sensors are arranged respectively upstream and downstream from a catalyst interposed in an exhaust emission system of an engine. Further, in such a construction, an air-fuel ratio feedback correction coefficient is set based on an output value of the upstream oxygen (O2) sensor arranged upstream from the catalyst and an air-fuel ratio is controlled such that an air-fuel ratio upstream from the catalyst becomes a target air-fuel ratio. Further, a dual O2 air-fuel ratio control system is proposed to achieve proper formation of an air-fuel ratio by correcting the air-fuel ratio feedback correction coefficient based on an output value of the downstream oxygen sensor arranged downstream from the catalyst.
Meanwhile, in such a dual O2 air-fuel ratio control system, when the respective oxygen sensors are deteriorated, response of the oxygen sensors is deteriorated. Therefore proper air-fuel ratio control is deteriorated.
Further, in the dual O2 air-fuel ratio control system, a deterioration of the catalyst is diagnosed by comparing outputs of the two oxygen sensors provided upstream and downstream from the catalyst. Therefore, when the respective oxygen sensors are deteriorated, accuracy of diagnosing the deterioration of the catalyst using the oxygen sensors is also deteriorated. Therefore it is necessary to detect the deterioration of the air-fuel ratio sensors.
At this occasion, since the upstream oxygen sensor is arranged upstream from the catalyst, an oxygen concentration in exhaust emission gas emitted from the engine is directly detected. Therefore, when a variation of the air-fuel ratio is brought about, the upstream oxygen sensor immediately reacts with the variation of the air-fuel ratio. Hence, the deterioration of the upstream oxygen sensor can comparatively easily be detected by monitoring the output of the upstream air-fuel ratio sensor when the variation of the air-fuel ratio is brought about.
In contrast thereto, since the downstream oxygen sensor is provided downstream from the catalyst, the downstream oxygen sensor detects the air-fuel ratio in emission gas after passing the catalyst. Therefore, even when the variation of the air-fuel ratio is brought about, the variation of the air-fuel ratio is smoothed by oxygen adsorption and separation by oxidation and reduction reaction of the catalyst or a storage effect of the catalyst and the downstream oxygen sensor detects the smoothed air-fuel ratio. Further, the storage effect of the catalyst is changed by the deterioration. Therefore, it is difficult to detect the deterioration of the downstream oxygen sensor per se from a state of reaction of the downstream oxygen sensor with respect to the variation of the air-fuel ratio of the engine.
In order to resolve the problem, a method is proposed to detect the deterioration of the downstream air-fuel ratio sensor which is difficult to be effected by influence of the catalyst. For example, in JP-U-03-037949, an output of an oxygen sensor downstream from a catalyst is detected with respect to a variation in an air-fuel ratio upstream from the catalyst before the catalyst is activated. Further, in JP-A-62-250351, deterioration is detected when an air-fuel ratio is changed more than a catalyst storage function as at fuel cut-off.
However, according to the method of detecting the deterioration of the oxygen sensor before activating the catalyst as in JP-U-03-037949, a condition of detecting the deterioration is limited to that in cold starting. Similarly, according to the method of detecting the deterioration of the oxygen sensor at fuel cut-off as in JP-A-62-250351, a condition of detecting the deterioration is limited to that at fuel cut-off. Particularly, in the case of the vehicle of an automatic transmission, fuel cut-off is hardly operated in running a city area. Therefore a frequency of executing deterioration detection is reduced.
In this way, in either of the methods, the executing condition is significantly limited. Therefore the frequency of detection is reduced. Further, even when the executing condition is established, the executing condition is under a transient condition. Therefore it is difficult to ensure detection accuracy.